In interpretation of signals received from a wide variety of receptors, (e.g., television cameras, x-ray scanning devices, radar), it is desirable to process the signals in such a manner as to produce a processed and filtered image capable of representation in a more specifically usable form, for example, as a visible display on a conventional video display terminal. It is also desirable to precisely control the properties of the individual aspects of the displayed image. These systems,referred to as hybrid-optical systems have the potential to gain an important role in signal processing due to their compactness, versatility and adaptability. Recently, high power, single-mode laser diode systems have become available enabling the useful operation of a certain class of hybrid-optical interferometric processors. In this type of processor, the diffracted and undiffracted orders of an acousto-optic cell are interfered. Due to the relative doppler frequency shift between the orders, the interference fringes are modulated at the doppler frequency. This typically would cause the destruction of fringe visibility when observed with time integrating detectors such as television cameras. To retain frame visibility, accordingly, optical source modulation is introduced which freezes the fringes to produce a high visibility output. Restoration of the fringe contrast can be achieved by shifting one of the two fields in frequency.
A second acousto-optical cell may be utilized and modulated at the same frequency as the original. However, this solution is not particularly desirable, due to the additional signal processing circuitry, acousto-optic cell, and associated optics which make this methodology unduly complex.
A continous wave gas laser may be utilized with external modulation sources. However, because of their bulk, high cost, and necessity for external modulation continuous wave gas lasers are not well suited to compact installations. In addition, CW gas lasers exhibit limitations in the number of wave lengths of emitted light which are available. Solid state laser diodes, on the other hand, offer a wide range of alternative wave lengths.
The present invention overcomes these shortcomings and provides a lower cost, efficient, compact and simplified method of improving fringe resolution.